The present invention relates to a touch panel to be placed on a screen of a liquid crystal display (LCD) or a cathode ray tube (CRT). The touch panel is used for inputting a position following an instruction displayed on the screen and viewed through the touch panel. The position is input through a user pressing the panel with a finger or pen.
Recently, a compact and lightweight portable terminal has been popular in the market, and a touch panel, which is used as an input device of the terminal, is required to be compact and lightweight. Further, the touch panel is required to be free from inconvenience even if the terminal is carelessly dropped.
Resistive touch panels have been widely used among the touch panels because the resistive touch panel has rather high detecting accuracy and can be manufactured at an inexpensive cost, and numbers of the touch panels are formed by combining plastic materials.
The conventional touch panel discussed above is described hereinafter with a relevant drawing. FIG. 4 is a sectional view of the conventional touch panel. The touch panel includes the following elements:
(a) An upper electrode sheet 1 including a flexible transparent film and a transparent electrode disposed on one of the faces of the sheet 1, the electrode being made of indium tin oxide (ITO) and the like;
(b) A lower transparent plastic board 2 having a transparent electrode;
(c) An insulating spacer 3A disposed on the periphery of both of the sheet 1 and board 2 for the transparent electrodes to face each other at a given distance; and
(d) A conical spacer 3, made of soft insulating material, disposed like dots at a given pitch on the transparent electrode of the lower-transparent-plastic board 2.
The conventional touch panel is usually placed on a screen of an LCD or a CRT, and an operator, who views an instruction displayed on the screen through the touch panel, presses the upper electrode sheet 1 at a given position from above with his/her finger or a pen. This action entails the upper electrode sheet 1 to bow downward partially, and the bowed portion of sheet 1 thus contacts with a corresponding portion of the transparent electrode disposed on the lower plastic board 2, so that the pressed position can be detected.
The transparent film of the upper electrode sheet 1 is usually made of polyethylene terephthalate because it is inexpensive and flexible enough for the input operation, and further, the ITO, which is formed on one of the faces of the film, is stabilized easily. The lower board 2 is usually made of polycarbonate because this material is inexpensive, highly transparent and processed easily.
Another conventional touch panel is shown in FIG. 5. This touch panel includes the following elements:
(a) An upper electrode sheet 1 including flexible transparent film and a transparent electrode disposed on one of the faces of sheet 1, the electrode being made of indium tin oxide (ITO) and the like;
(b) A lower electrode sheet 4 having a transparent electrode;
(c) An insulating spacer 3A disposed on the periphery of both sheets 1 and 4 for the transparent electrodes to face each other at a given distance;
(d) A transparent holder board 5 made of a rigid transparent plastic board and disposed under the lower face of the sheet 4, where no electrode is formed;
(e) A transparent bonding layer 6 for bonding the board 5 to the sheet 4; and
(f) A conical spacer 3, made of soft insulating material, disposed like dots at a given pitch on the transparent electrode of the lower board 2.
This touch panel operates similarly to the panel shown in FIG. 4 previously described, i.e., the touch panel is placed on a screen, and an operator presses the upper electrode sheet 1 at a given position from above. This action entails the upper electrode sheet 1 to bow downward partially, and the bowed portion of sheet 1 thus contacts with a corresponding portion of the transparent electrode disposed on the lower plastic board 2, so that the pressed position can be detected.
In this touch panel, the electrode sheets 1 and 4 are made of polyethylene terephthalate, and the holder board 5 is made of polycarbonate because of the same reason discussed above.
Both of the conventional touch panels discussed above employ polyethylene terephthalate combined with polycarbonate. These two materials are largely different in coefficients of linear expansion. Therefore, when the touch panel is exposed in a severe temperature or moisture environment, dimensions of members of the panel change differently from each other, which may cause waviness, i.e., distortion on the film, particularly in the case that the film is thin. This phenomenon lowers visibility of the touch panel and makes the distance between the upper and lower transparent electrodes narrower if the waviness is large, thus causing an unexpected short between both of the electrodes.
A reliable touch panel of the present invention features the following advantages. Even when being exposed in a severe temperature or moisture environment, the touch panel does not has waviness produced on an upper or lower electrode sheet. The panel thus hardly has the electrodes short-circuit and maintains excellent visibility.
The touch panel includes a transparent film, an upper electrode sheet having a first transparent electrode disposed on the transparent film, and a lower electrode board having a rigid transparent plastic board and a second transparent electrode. The second transparent electrode is disposed on the transparent plastic board and faces the first transparent electrode. The first transparent film and the transparent plastic board are made of respective materials containing an identical main component to each other.